Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2001-12-14
2004-05-04
Glick, Edward J. (Department: 2882)
Optical waveguides
With optical coupler
Particular coupling function
C385S043000, C385S050000, C372S006000, C372S069000, C359S341300
Reexamination Certificate
active
06731837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The technical field is fiber amplifiers and lasers and specifically devices for optically pumping these in order to create a population inversion within an active medium of such amplifiers and/or lasers.
2. Description of Related Art
Devices for optically pumping fiber amplifiers and lasers are currently being used in time division and wave length division multiplex fiber communication systems, free space communication systems, remote measurement and sensing, scientific and laboratory experimentation and other applications.
One problem in the design of fiber amplifiers and lasers is to administer pump light to the active medium of the fibers with a power intensity sufficient to produce a reasonable gain.
Transversal pumping schemes popular with lasers in which the active medium has cross section dimensions of at least several millimeters are not straightforwardly applicable to fiber amplifiers and lasers. Due to the small cross section, overall absorption of transverse pumping light in the fiber is low, resulting in poor efficiency. On the other hand, absorption cannot be increased by increasing the concentration of the active species within the host material of the active medium since this may lead to thermal problems, to a narrowing of the gain vs. wavelength distribution and to other undesirable effects.
Accordingly, setups for longitudinally pumping fiber amplifiers and lasers have been studied. For example, in L. Goldberg, I. P. Koplow, D. Kliner, “Highly efficient 4-W Yb-doped fiber amplifier pumped by a broad stripe laser diode”, Optics Letters, volume 24, pages 673 to 675, 1999, it has been shown that longitudinal pumping of a double cladding fiber structure is an effective approach for constructing high power fiber amplifiers and lasers. Similar pumping setups are known from U.S. Pat. No. 4,815,079, J. D. Minelli et al., “Efficient cladding pumping of an Er fibre”, paper Th.L.1.2, Proceedings of 21
st
European Conference on Optical Communications, Brussels 1995, and P. Bousselet et al., 26 dBm output power from an engineered cladding-pumped Yb-free EDFA for L-band WDM applications”, paper WG5-1, Optical Fiber Conference San Francisco 2000.
Since the typical dimension of the inner cladding of such a fiber is 90 to 150 &mgr;m, non-diffraction limited emission from high power broad area laser diode pumps can be efficiently coupled into such fibers. A 90 &mgr;m wide broad stripe laser diode can generate an output power of 2 to 4 W at 89, 915 or 980 nm with long operating life.
The pump absorption coefficient of a double cladding fiber is inversely proportional to the ratio between the inner cladding area and the core area, which is typically in the range of 200-500. Because of this large ratio it is necessary to use very high dopant densities in order to achieve an adequate pump absorption coefficient, which is required to construct a reasonably short amplifier. Excessively long fiber amplifiers are not desirable because of background propagation losses in the gain fiber, increased cost, and signal distortion and interaction caused by nonlinear effects in the fiber core.
High doping densities can be used with certain dopants, such as Yb, where a concentration of 10
4
to 2×10
4
ppm results in a typical cladding absorption coefficient of 1-3 dB/m at the peak Yb absorption wavelength of 975 nm. Erbium is more desirable as an active species for fiber amplifiers due to its usable gain band of 1530 to 1600 nm within the maximum transmission window of common fiber-glass compositions. However, it has been shown by P. Myslinski, D. Nguyen and J. Chrostowski, “Effects of Concentration on performance of Erbium-doped Fiber Amplifiers”, J. Lightwave Tech., v. 15, pp. 112-119 (1997) that in case of Er the concentrations must be kept at least ten times smaller (typically below 900 ppm) than these values to avoid concentration quenching effects which significantly reduce amplifier quantum efficiency.
A known technology for circumventing this deficiency is Er/Yb co-doping (cf. J. E. Townsend, W. L. Barnes, K. P. Jedrzejewski, S. G. Grabb, “Yb-sensitized Er Doped Silica Optical Fiber with High Transfer Efficiency and Gain”, Electronics Lett. v. 27, pp. 1958-1959, (1991) where high Yb concentration (10-20 times that of Er) is used to achieve high pump absorption, and energy absorbed by Yb is efficiently transferred to Er ions. To achieve this efficient transfer, however, the core glass composition needs to have a large P
2
O
5
content. Such gain fibers are difficult to fabricate with low background loss and high quantum efficiency. In addition the presence of P
2
O
5
in the host glass causes substantial narrowing of the gain vs. wavelength distribution, and results in a small signal gain distribution that is much less uniform than that of amplifiers with Er-doped silica host glass cores. This gain non-uniformity makes the Er/Yb co-doped amplifiers unsuitable for WDM applications that require relatively flat gain distribution across a wide wavelength span.
An attractive method for constructing high power Er-doped amplifiers is to use double cladding structures having a small-inner-cladding. Such devices are described in J. D. Minelli, Z. J. Chen, R. L. Laming, J. D. Caplen, “Efficient cladding pumping of an Er fibre”, paper Th.L.1.2, Proceedings of 21
st
European Conference on Optical Communications, Brussels 1995, and in P. Bousselet, M. Bettiati, L. Gasca, M. Goix, F. Boubal, C. Sinet, F. Leplingard, D. Bayart, “26 dBm output power from an engineered cladding-pumped Yb-freeEDFA for L-band WDM applications”, paper WG5-1, Optical Fiber Conference, San Francisco, 2000.
A small cladding, and a correspondingly small cladding-to-core area ratio make it possible to achieve high pump absorption with reasonably low concentrations. A maximum Er concentration of approximately 900 ppm, possible with a host glass with a large Al
2
O
3
content, would result in a core absorption coefficient of approximately 20 dB/m. In a double cladding fiber with an area ratio of A(cladding)/A(core)=25-910, this would result in a cladding absorption coefficient of 0.8-0.2 dB/m. Assuming 90% pump absorption, these absorption coefficients are sufficiently high to construct efficient amplifiers with a length of 12 to 60 m, comparable to that of conventional, core-pumped amplifiers. Assuming a typical core diameter of 5 &mgr;m these cladding-to-core area ratios require an inner cladding diameter of 25-50 &mgr;m. The upper limit of core diameter, as dictated by bend losses and mode mismatch losses when fusion splicing to conventional transmission fibers, is represented by a 0.1 NA core of 9 &mgr;m, corresponding to a cutoff wavelength of approximately 1.45 &mgr;m. Such a core size would allow larger cladding diameters and/or larger absorption coefficients.
In addition to achieving sufficiently high pump absorption in double cladding fibers, another issue that must be addressed is whether a sufficiently high pump intensity can be achieved to produce a population inversion level required for high gain across the entire usable gain band of the active species. For Er, the threshold pump intensity, required for a 50% population inversion (50% of the ion population is in the upper state
4
I
13/2
) is given by I
th
=h&ngr;/st=9 kW/cm
2
, where h&ngr; is the photon energy, s=2×10
−21
cm
2
@980 nm is the absorption cross-section and t~9 ms is the upper state lifetime. To achieve high gain and low noise performance in the amplifier, a population inversion of >80% is desirable, requiring a pump intensity that is approximately 5 I
th
, or 50 kW/cm
2
. For a cladding pumped fiber amplifier with a 50 &mgr;m cladding diameter, this requires a local pump power of 1 W. Since in an efficient amplifier the fiber needs to be sufficiently long to allow almost complete pump light absorption, the local pump intensity decreases exponentially as a function of position, causing a decrease in the local population inversion with distance fr
Goldberg Lew
Le Flohic Marc
Andrews & Kurth LLP
Glick Edward J.
Keopsys, Inc.
Suchecki Krystyna
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